feat: migrate website-creator from Next.js+Payload to Astro+Tina CMS

Major changes:
- Replace Payload CMS with Tina CMS (self-hosted)
- Add Astro DB for consent logging (PDPA compliant)
- Update Tailwind v3 to v4 (@tailwindcss/vite plugin)
- Add astro-tina-starter template
- Rewrite consent template for Astro (ConsentBanner.astro, Astro DB, Nano Stores)
- Add install-tina-backend.sh for self-hosted Tina per customer
- Rename convert-astro.sh to migrate-tina.sh
- Add AGENTS.md template for generated websites
- Delete all Payload/Next.js files

Technical updates:
- Astro DB using defineDb with eq operators for queries
- Tailwind v4 with @theme block
- Tina CMS local development mode
- Proper Astro API routes for consent

Research-verified with official documentation (April 2026)

skills/seo-multi-channel/scripts/templates/blog.yaml

@@ -160,12 +160,12 @@ output:
   encoding: "utf-8"
   line_endings: "unix"
   
-  astro_integration:
-    content_collection: "src/content/blog"
-    language_folders:
-      thai: "(th)"
-      english: "(en)"
-    image_folder: "public/images/blog/{slug}/"
+  payload_cms_integration:
+    collection: "posts"
+    language_prefix:
+      thai: "th/"
+      english: ""
+    image_collection: "media"
     
   publishing:
     auto_publish: "optional (user_choice)"